1. Field of the Invention
The present invention relates to an optical add/drop multiplexer device for extracting or inserting optical signals of a specific wavelength at a specific node in constructing a wavelength division multiplexing network in a transmission system for long distance optical fiber communication, optical subscriber system communication, optical LAN, optical CATV, etc.
2. Description of the Background Art
In a recent trend for broadband multimedia communication, in addition to a demand for drastic increase of a transmission capacity of the optical communication system, there is also a demand for improvement in a flexibility of the optical communication system by way of parallel implementation or network implementation, for example. The wavelength division multiplexing (WDM) scheme is a transmission scheme that can satisfy the above condition, so that it is an important technology which is applicable to a wide range of transmission systems ranging from a long distance transmission system to a subscriber access system. For instance, in the WDM scheme, it is possible to extract or insert only a part of optical signals at a specific node so as to realize an effective utilization of a transmission path, unlike a conventional single wavelength communication.
In the WDM scheme, in practice, there is a need to provide an optical add/drop multiplexer (ADM) device having a function for extracting only those optical signals which have a specific wavelength by using a wavelength selection filter, or a function for inserting only those optical signals which have a specific wavelength, at a point where transmission paths intersect.
An operation of such a conventional optical ADM device will now be described with reference to FIG. 1. In a state where optical signals in four channels having different wavelengths .lambda..sub.1 to .lambda..sub.4 are transmitted from a ground 91, when an optical ADM device 90 is provided between the ground 91 and another ground 93 as shown in FIG. 1, it becomes possible to extract or insert only those optical signals which have a specific wavelength .lambda..sub.2 at an intermediate ground 92, without terminating a cable.
For this type of optical ADM device, it is possible to use a configuration as shown in FIG. 2 in which a fiber grating filter 94 is used in combination with optical circulators 95 and 96 (see C. R. Giles and V. Mizrahi, "Low-Loss ADD/DROP Multiplexers for WDM Lightwave Networks", paper ThC201, in Technical Digest of IOOC '95), or a configuration in which a plurality of dielectric multilayer filters are combined.
In addition, there are also reports of waveguide type wavelength selection filters using semiconductors or dielectrics. The wavelength selection filters using semiconductors include a directional coupler filter using a grating (see R. C. Alferness, T. L. Koch, L. L. Buhl, F. Storz, F. Heismann, and M. J. R. Martyak, "Grating-Assisted InGaAsP/InP vertical codirectional coupler filter", Applied Physics Letter, Vol. 55, pp. 2011-2013 (1989), or H. Sakata and S. Takeuchi, "Grating-Assisted Directional Coupler Filters Using AlGaAs/GaAs MWQ Waveguides", IEEE Photonics Technology Letters, Vol. 3, pp. 899-901 (1991)), and a notch filter using a grating (see J.-P. Weber, B. Stoltz, M. Dasler, and B. Koek, "Four-channel Tunable Optical Notch Filter Using InGaAsP/InP Reflection Gratings", IEEE Photonics Technology Letters, Vol. 6, No. 1, pp. 77-79 (1994)).
FIG. 3 shows the directional coupler filter as disclosed by Alferness et al., while FIGS. 4A ad 4B show the notch filter as disclosed by Weber et al.
Now, in the WDM scheme, there is a need to increase a number of transmission wavelengths in order to increase a transmission capacity, and as a wavelength interval becomes narrower, a filter characteristic of an optical ADM device is also required to have a narrower bandwidth. In addition, from a point of view of improving the flexibility of the network, it is desirable to be able to freely select a channel (wavelength) to be added/dropped at a specific node or to change a number of channels to be added/dropped at a specific node. For example, in the WDM scheme using four channels with wavelengths .lambda..sub.1 to .lambda..sub.4 as shown in FIG. 1 in which the optical signals with the wavelength .lambda..sub.2 can be added/dropped at the ground 92, it would be desirable if it is also possible to add/drop the wavelength .lambda..sub.3 instead of the wavelength .lambda..sub.2, or to add/drop two channels of .lambda..sub.2 and .lambda..sub.3 instead of just one channel. When the optical ADM device having such a function is available, it will be possible to flexibly deal with a change of channel demand so that it will be useful for the restoration in a case of emergency as well, and it will be possible to construct a dynamic WDM network.
In the conventional optical ADM device using a fiber grating or a dielectric multilayer filter, it is possible to obtain the filter characteristic with a narrow bandwidth, but this in turn requires optical components such as optical circulators and 3dB couplers so that the device itself becomes quite large in size, which is both ineconomical as well as problematic from a point of view of reliability.
In addition, it is difficult to change the wavelength to be selected by the filter in the conventional optical ADM device. Consequently, in order to realize selections of a plurality of channels, it is necessary to arrange a plurality of optical ADM devices in parallel or in series, but this in turn requires a complicated configuration.
On the other hand, in the waveguide type wavelength filter using semiconductors, it is possible to realize a configuration in which currents can be injected into a filter region so that the reflectivity of the filter region can be controlled by a change of the injected currents, and by means of this configuration it is possible to change the wavelength to be selected by the filter.
However, in the directional coupler type wavelength filter mentioned above, it is necessary to elongate the coupling length in order to obtain the filter characteristic with a narrow bandwidth, and this in turn gives rise to a problem of a large size of the device. For instance, in the exemplary configuration disclosed by Alferness et al., the 3dB width of the filter characteristic becomes 6.5 nm for the coupling length of 1 mm. In addition, in order to control the filter characteristic such as the half width, an interval between two optical waveguides or a grating shape must be controlled rigorously so that there is a problem in that it becomes increasingly difficult to manufacture the device when the coupling is made stronger.